If you look at the early history of RPGs, one thing you'll notice is that it tends to focus on genres that involve a great deal of world-building; namely, fantasy and SF. D&D, Traveller, Tekumel, Metamorphosis Alpha, Runequest, and so on. Until Champions came out in 1982, V&V and Superhero2044 were pretty much it for supers, and I don't recollect any horror games in existence before Call of Chtulhu came out.
Ergo, I think that there is something about world-building genres that draws the kind of people who are attracted to the RPG concept. I mean, how many times have you had some gamer eager to explain to you "how magic works in *my* world"?
Now, in looking at fantasy and SF in particular, an important difference is that SF tends to focus more on *concepts* than it does on characters. Look at Asimov's Foundation; that whole series is really the exploration of various theories. It's not really about any one particular character (save Hari Seldon, but he's almost more like a force of nature). Tolkien, otoh, can write a sprawling epic that opens up a vastly imgained world, yet it's still very much about a couple of hobbits and their buddies. Granted, there are exceptions to this (Trek, SW), but I think it can be identified as a trend.
Ergo, fantasy appeals to the world-building needs of those attracted to it, yet still is primarily focused on individuals; the needs of both the GM and the players are being satisfied.
Then we add the aformentioned research needs. You look at any general RPG book on SF (e.g., Star HERO, GURPS Space), and you'll notice a lot of pages dedicated to familiarizing the reader with cosmology, planetology, and physics. SF demands at least a veneer of scientific verisimilitude (e.g., the technical mumbo-jumbo featured so prominently on Trek).
Fantasy (and space opera, like Star Wars or Flash Gordon) does not require this. Sure, a knowledge of mythology and history is useful, but it's not *necessary* to the same degree. The the strong tropes of post-Tolkien fantasy and the implied setting of D&D, in particular, also takes on a lot of the world-creating burden. The GM, then, is simply required to describe what *is*, not so much *how it all works*. You're not starting from scratch so much as rearranging the peices.
(This isn't to say that fantasy world-building is easy; I disputed that in my post about the supers genre. But world-building itself is the draw I'm talking about, and world-building is simply not as big a part of genres like supers, horror, or modern action.)
Lastly, I think that the kind of people who tend to like RPGs tend to be avid readers. The kind of people who, when faced with the prospect of reading 900 pages of rules, say "Cool!" I.e., they're in a literary/reader mindset. SF has morphed in recent decades from being a literary genre to one generally conveyed via television or film. Ditto horror, barring Stephen King (and even then, most of his works have been made into movies), and the ascendance of Anne Rice (which, poof!, inspired V:TM). Supers is born of comic books and animated movies; leaving aside arguments about the literary merit of comics, we're again not talking about traditional literature, or traditional "readers". Fantasy, however, seems to rarely escape the confines of the printed page, at least with any sort of quality. The recent LOTR films --and, stretching our deifnition of fantasy a bit, Harry Potter-- are the glaring exceptions to this.
Ergo, added to the observations above, I think that fantasy, by it's very nature, attracts the book-lover, and book-lovers, in turn, tend to be the ranks from which gamers tend to be born.
Given all of this, it's no surprise to me that it was fantasy that inspired Arneson and Gygax to create the hobby, and fantasy that conitnues to dominate it. It simply has all the elements that make it well-suited to the concept of the RPG and the kind of people attracted to it.
However, I think that there has been some evolution. In 1974, being a wargaming/fantasy/SF geek was all about being an avid reader. Now, though, we're seeing avid consumpiton of movies and TV informing the average gamer geek. Voila! We see more successful games based on film/TV licenses, the continued use of "cinematic" as a desireable trait for an RPG system, and a general broadening of subject matter inspiring the hobby.